Process and apparatus for recovering sulphur from a gas stream containing sulphide

ABSTRACT

Oxygen or oxygen-enriched air is employed to support combustion in furnaces ( 16 ) and ( 26 ) of part of the hydrogen sulphide content of a first feed gas stream. Sulphur vapour is extracted in condenser ( 32 ) from the resulting gas mixture so as to form a sulphur vapour depleted gas stream. The sulphur vapour depleted gas stream is passed into a catalytic reduction reactor ( 40 ) in which all the residual sulphur dioxide is reduced to hydrogen sulphide. This reduced gas mixture has water vapour extracted therefrom in a quench tower ( 52 ). The resulting water vapour depleted gas stream flows to a Claus plant for treatment typically together with a second feed gas steam comprising hydrogen sulphide. Employing both furnaces ( 16 ) and ( 26 ) makes it possible to obtain effective conversions to sulphur of the hydrogen sulphide in the feed gas without having the recycle any of the water vapour depleted gas.

BACKGROUND OF THE INVENTION

This invention relates to a method of treating a feed gas streamcomprising hydrogen sulphide.

Gas streams comprising hydrogen sulphide are formed for example as wastegases in an oil refinery or gas refinery operation. In view of theirhydrogen sulphide content, these gas streams cannot be discharged to theatmosphere without first being treated so as to remove almost all thehydrogen sulphide.

A standard method of treating such a gas stream serves to recoversulphur therefrom is by the Claus Process. Conventional Claus processesare described in the introductory paragraphs of EP-A-565 316.

EP-A-565 316 discloses a process in which in a first reactor a part ofthe hydrogen sulphide content of a feed stream comprising hydrogensulphide is oxidised to sulphur dioxide, and so formed sulphur dioxideis reacted with residual hydrogen sulphide to form sulphur vapour andwater vapour. A partially reacted gas stream including sulphur vapour,water vapour, residual hydrogen sulphide and residual sulphur dioxide iswithdrawn from the furnace. A sulphur condenser is employed to extractsulphur vapour from the partially treated gas stream so as to form asulphur vapour depleted gas stream. At least part of the sulphurdepleted gas stream is sent to a further reactor in which its sulphurdioxide content is reduced to hydrogen sulphide. Water vapour isextracted from the resulting reducer gas stream. The watervapour-depleted gas stream is then preferably recycled to the furnace. Apurge stream is taken from a chosen position in the above describedcycle and is subjected to further treatment so as to render it fit fordischarge to the environment. The purpose of the recycle is to obtain avery high effective conversion of hydrogen sulphide to sulphur vapour inthe furnace and thereby facilitate the attainment of a total conversionefficiency which is sufficient to meet any prevailing environmentalstandard.

By using pure oxygen (or air highly enriched in oxygen) the size of theinitial furnace may be kept down. However, the advantages in sizereduction of the initial purge gas made possible by the use of pureoxygen (or oxygen highly enriched in air) as the oxidant arecounteracted by the recycle of gas to the furnace. Although EP-A-0 565316 further discloses that the recycle can be omitted, this is statednot to be preferred as it has an adverse effect on the effectivepercentage conversion of hydrogen sulphide to sulphur in the furnace.

One solution to this problem suggested in EP-A-565 316 is to employ anamine separation unit to concentrate the recycle stream in hydrogensulphide. Such amine separation units, however, tend to be particularlycostly, even if only of a small size.

It is an aim of the method according to the invention to provide analternative solution to this problem which does not necessitate arecycle.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method oftreating a feed gas stream comprising hydrogen sulphide, comprising thesteps of:

-   -   (a) in at least two furnaces in series oxidising to sulphur        dioxide a part of the hydrogen sulphide content of at least one        feed stream comprising hydrogen sulphide and reacting thus        formed sulphur dioxide with residual hydrogen sulphide to form        sulphur vapour and water vapour;    -   (b) withdrawing from the downstream furnace a partially reacted        gas stream including sulphur vapour, water vapour, residual        hydrogen-sulphide and residual sulphur dioxide;    -   (c) extracting in a sulphur condenser, sulphur vapour from the        partially treated gas stream so as to form a sulphur vapour        depleted gas stream;    -   (d) catalytically reducing with hydrogen to hydrogen sulphide        the sulphur dioxide and any sulphur vapour content of the        sulphur vapour depleted gas stream so as to form a reduced gas        stream    -   (e) extracting water vapour by condensation from the reduced gas        stream so as to form a water vapour depleted gas stream; and    -   (f) feeding without cycle the water vapour depleted gas to at        least one Claus plant for recovering sulphur from hydrogen        sulphide comprising at least one further furnace for the        oxidation of hydrogen sulphide to sulphur dioxide and reaction        of resulting sulphur dioxide with residual hydrogen sulphide, a        first further sulphur condenser, and a plurality of stages of        catalytic reaction to form sulphur vapour, there being a second        further sulphur condenser downstream of each stage of said        catalytic reaction, and thereby extracting further sulphur        vapour, wherein:        a gas containing at least 80% by volume of oxygen molecules is        employed to support combustion in step (a); the water vapour        depleted gas stream contains at least 40% by volume of hydrogen        sulphide; and in step (f) the catalytic reaction is a Claus        reaction between hydrogen sulphide and sulphur dioxide or a        selective oxidation of hydrogen sulphide to sulphur, or both        reactions.

The method according to the invention make it possible to achievewithout recycle of the water vapour depleted gas, a high percentageconversion of hydrogen sulphide to sulphur upstream of the hydrogenationreactor while still obtaining a water vapour depleted gas streamsufficiently concentrated in hydrogen sulphide to enable it to bereadily treatable in the Claus plant. Accordingly, if the Claus plant isan existing one, very high levels of uprating, typically at least 250%,and sometimes much higher can be achieved. A further advantage of themethod according to the present invention is that it has such a heatbalance that it can be a net exporter of high pressure, super heated,steam. This steam may be expanded in a turbo-expander which drives anelectrical generator. Accordingly, electricity may be generated.

Preferably, from 80 to 90% of the hydrogen sulphide is converted tosulphur vapour upstream of the hydrogenation reaction in step (d) of themethod according to the invention. This facilitates the achievement of ahigh uprating of the Claus plant.

It is preferred that the mole ratio of hydrogen sulphide to sulphurdioxide in the sulphur vapour depleted gas stream is in the range 4:1 to10:1. This ratio may be achieved by appropriately limiting orcontrolling the rate of oxygen flow to the furnaces.

Preferably, the flow rate of oxygen to the upstream furnace is in therange of M to N where M=(0.8 a+b+0.16c) and N=(a+b+0.22c) where

-   -   a is the stoichiometric rate of supply of oxygen for the        complete oxidation to nitrogen and water vapour of any ammonia        in the feed gas stream;    -   b is the stoichiometric rate of supply of oxygen for the        complete combustion of any hydrocarbon(s) present in the feed        gas stream; and    -   c is the stoichiometric rate of oxygen supply required for the        complete oxidation to sulphur dioxide and water vapour of        hydrogen sulphide in the feed gas stream.

Conventionally, in Claus processes, oxygen for reaction of hydrogensulphide is supplied at a rate approximately equal to 0.33c. Byoperating with substantially less oxygen the hydrogen sulphide tosulphur dioxide ratio in the first furnace is kept up thereby favouringthe reduction of sulphur dioxide to sulphur vapour. Accordingly, veryhigh levels of conversion of sulphur dioxide to sulphur can be achievedin the two furnaces in step (a) of the method according to theinvention. Thus, any requirement for an external supply of hydrogen maybe limited.

The rate of supply of oxygen to the second or downstream furnace in step(a) (when just two furnaces are employed in step (a)) is in the range Pto Q, where P=0.8 d+e+0.16 f and Q=d+e+0.22f, wherein:

-   -   d is the stoichiometric rate of oxygen required for the total        oxidation to nitrogen and water vapour of any ammonia entering        the downstream furnace (preferably no ammonia enters the        downstream furnace);    -   e the stoichiometric rate of oxygen supply required for the        total combustion to carbon dioxide and water vapour of any        hydrocarbons entering the downstream furnace (preferably no        hydrocarbons enter the downstream furnace); and    -   f is the stoichiometric rate of oxygen supply required for the        complete combustion of hydrogen sulphide entering the downstream        furnace to water vapour and sulphur dioxide.

Because the oxidation of hydrogen sulphide is strictly limited in thefurnaces employed in step (a) of the method according to the invention,the amount of heat generated in each individual furnace is limited,thereby making unnecessary the introduction of special coolants orrecycle streams into each furnace for the purposes of temperaturelimitation. Nonetheless, the effluent gas stream from the upstreamfurnace is preferably cooled, for example, in a waste heat boiler,upstream of the downstream furnace. This gas flow is preferably cooledto a temperature in the range of 300 to 500° C.

If desired, when two furnaces are employed in step (a) of the methodaccording to the invention, sulphur vapour may be condensed between theupstream furnace and the downstream furnace. (If desired, the resultinggas stream may be reheated upstream of the downstream furnace to atemperature at which reaction between oxygen and hydrogen sulphide isautogenous.) Alternatively sulphur vapour can be allowed to pass fromthe upstream to the downstream furnace without there being intermediatesulphur condensation.

Sufficient hydrogen for the complete reduction to hydrogen sulphur andany residual sulphur vapour in the sulphur vapour depleted gas streammay sometimes be formed in situ by the thermal cracking of both hydrogensulphide and ammonia during step (a) of method according to theinvention. If desired, an external source of hydrogen can be provided toensure that there is always an adequate amount of hydrogen available forreduction of the sulphur dioxide and any residual sulphur vapour.

Any known catalyst of the reaction between hydrogen and sulphur dioxideto form water vapour and hydrogen sulphide may be used in the catalytichydrogenation stage.

The inlet temperature to the catalytic hydrogenator is preferably in therange of 200° C. to 400° C. The further sulphur depleted gas stream ispreferably reheated intermediate the sulphur condensation step and thecatalytic hydrogenation step.

If desired, the catalytic hydrogenation step may be performed withexternal cooling so as to limit the size of any temperature increasethat takes place as a result of the exothermic reduction reaction. Theexternal cooling is preferably formed by adding steam to the sulphurvapour depleted gas stream.

The water condensation step is preferably performed by direct contact ofthe reduced gas stream with water, the reduced gas stream being cooledby indirect heat exchange intermediate the catalytic reduction step andthe water condensation step.

Preferably the resulting water vapour depleted gas leaves the watercondensation step as a gas saturated in water vapour at a temperature inthe range of 30° C. to 50° C. As a result, typically at least 85% of thewater vapour present in the reduced gas stream is removed in the watercondensation step.

A water vapour depleted gas stream containing at least 40% by volume ofhydrogen sulphide is generally readily treatable in conventional Clausplants that use air to support combustion. When the hydrogen sulphideconcentration is of the order of 40% by volume, some oxygen-enrichmentof air used to support combustion in the further furnace or furnaces maybe used, or some of the water vapour depleted gas may be by-passed to adownstream region of the further furnace or furnaces. Thus the Clausplant may be a conventional Claus plant, and hence the upstream integers(a) to (d) of the apparatus according to the invention may beretro-fitted to a subsisting Claus plant so as to uprate it, typicallyby at least 250%. The feed to the said Claus plant may be supplementedwith a second feed stream containing more than 40% by volume of hydrogensulphide. Alternatively, or in addition, a gaseous oxidant containing atleast 80% by volume of oxygen, preferably at least 90% by volume ofoxygen, may be used to support combustion in the further furnace orfurnaces. Otherwise, combustion may be supported by air unenriched inoxygen or oxygen-enriched air (or separate streams of air and oxygen oroxygen-enriched air) containing less than 80% by volume of oxygen.

Preferably two or three stages of catalytic reaction between hydrogensulphide and sulphur dioxide are employed in the said Claus plantemployed in step (f) of the method according to the invention dependingon the overall conversion required.

Preferably, the said Claus plant additionally includes at its downstreamend a so-called “tail gas clean up unit” which may typically include, inseries, a water condenser, a unit for the reduction of sulphur dioxideto hydrogen sulphide, and a unit for absorption of hydrogen sulphidefrom the tail gas. The absorbent is preferably an amine which isselective for hydrogen sulphide.

If in the method according to the invention a plurality of Claus plantsin parallel is employed, the Claus plants may share a common tail gasclean up plant.

The furnaces in step (a) of the method according to the invention arepreferably operated at a pressure in the range of 1 to 2 bar absolute.The further furnace is preferably operated at a similar pressure.Preferably, in order to facilitate flow of the water vapour depleted gasstream into the further furnace, a flow of steam is employed so as toraise the pressure of the reduced gas stream. Preferably the steam isadded to the reduced gas stream intermediate the hydrogenation and waterextraction steps of the method according to the invention. Otherpositions are, however, possible for the addition of the steam. One ormore eductors may be used for this purpose. Using such eductors makes itunnecessary to employ a fan or other rotary device for feeding the watervapour depleted gas stream into the plant or plants employed in step (e)of the method according to the invention. Further, the introduction ofthe steam upstream of the water vapour condensation step has the resultthat the total water vapour content of the water vapour depleted gasstream need not be increased and therefore has no detrimental effect ofthe operation of the downstream Claus plant or plants.

If desired, particularly if the hydrogen content of the water vapourdepleted gas stream is greater than say, 10% by volume, the hydrogen maybe separated from the water vapour depleted gas stream upstream of theClaus plant or plants.

The Claus plant preferably employs gaseous oxidant containing at least80% by volume of oxygen molecules in the further furnace or furtherfurnaces.

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to the invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a generalised schematic flow diagram of a plant 2 forrecovering sulphur from a gas stream containing hydrogen sulphidecomprising a set 4 of retrofitted units; a main Claus plant 6, and atail gas clean up unit 8;

FIG. 2 is a schematic flow diagram illustrating a configuration of units2 for use in the plant shown in FIG. 1.

FIG. 3 is a schematic flow diagram illustrating a first alternativeconfiguration of units 2 for use in the plant shown in FIG. 1;

FIG. 4 is a schematic flow diagram illustrating a second alternativeconfiguration of units 2 for use in the plant shown in FIG. 1;

FIG. 5 is a schematic flow diagram of the main Claus plant 6 shown inFIG. 1; and

FIG. 6 is a schematic flow diagram of the tail gas clean up unit 8 shownin FIG. 1.

The drawings are not to scale. Like parts in different Figures of thedrawings are indicated by the same reference numerals.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 of the drawings, a sulphur recovery plant 2comprises in sequence an upstream set of units 4 for performing steps(a) to (e) of the method according to the invention, a main Claus plant6 for performing step (f) of the method according to the invention, anda tail gas clean-up plant 8 for cleaning the effluent gas from the Clausplant 6.

Referring now to FIG. 2 of the drawings, a hydrogen sulphide containingfeed gas stream typically comprising hydrogen sulphide, carbon dioxideand water vapour, and sometimes additionally including ammonia and/orone or more hydrocarbons is fed from a pipeline 12 to a burner 14 whicheither fires axially into a thermal reactor in the form of arefractory-lined furnace 16, through one end wall 18 thereof, or firestangentially through a side wall at a position close to the end wall 18,typically at right angles to the axis of the furnace 16. The feed gastypically contains at least 70% by volume of combustibles. If the feedgas stream is a waste stream of an oil refinery it may be an acid gas(sometimes referred to as “amine gas”), or a mixture of amine gas withsour water stripper gas. The hydrogen sulphide containing feed gasstream is supplied to the burner 14 typically at a temperature in therange of 0° C. to 90° C., preferably 10° C. to 60° C., and is typicallynot preheated upstream of the furnace 16. The burner 14 is suppliedseparately from a pipeline 20 with a stream of commercially pure oxygenor a stream of air highly enriched in oxygen. In either case, the molefraction of oxygen in the gas that is supplied along the pipeline 20 isat least 0.8. Indeed, the oxygen stream can typically contain at least90% by volume of oxygen and may be separated from air by, for example,pressure swing adsorption or by fractional distillation, the latterseparation method being able to produce oxygen at a purity in excess of99%. A purity in excess of 99% is particularly preferred. By operationof the burner 14 a part of the hydrogen sulphide content of the firstfeed gas stream is burned in the furnace 16.

The rate of flow of oxygen or the oxygen content of an oxygen-enrichedair along the pipeline 20 is in the range of M to N, where M=(0.8a+b+0.16c) and N=(a+b+0.22c), and where a is the stoichiometric flow ofoxygen required for the complete oxidation to nitrogen and water vapourof any ammonia present in the feed, b is the stoichiometric flow rate ofoxygen required for the complete oxidation to carbon dioxide and watervapour of any hydrocarbons present in the feed, and c is thestoichiometric flow rate of oxygen required for the complete oxidationto water vapour and sulphur dioxide of the hydrogen sulphide content ofthe feed gas stream. In conventional terms, therefore the burner 14operates with a relatively oxygen poor flame. Nonetheless, high flametemperatures, typically with a core temperature of over 2000° C., can beachieved without causing the outlet temperature of the furnace 16 toexceed 1600° C.

In addition to the abovementioned reactions, there is also thermaldissociation of a part of the hydrogen sulphide into hydrogen andsulphur vapour and some thermal dissociation of ammonia into hydrogenand nitrogen. Employing a combustion supporting gas rich in oxygenfacilitates thermal dissociation (also known as thermal cracking) ofhydrogen sulphide and ammonia particularly if high temperature zone(s)at a temperature of, over, say 2000° C. are created. Various otherreactions may also take place in the furnace 16 such as the formation ofcarbon monoxide, carbon oxysulphide and carbon disulphide.

In operating the burner 14 and the furnace 16, care should of course betaken to avoid damage to the refractory lining. The angle and positionof entry of the burner 14 into the furnace 16 and the flameconfiguration are chosen so as to avoid such damage. The thermaldissociation of hydrogen sulphide has a cooling effect which can betaken into account in selecting the position and angle of entry of theburner 14.

As a result of the reactions that take place in the furnace 16, aneffluent gas stream typically comprising hydrogen sulphide, sulphurdioxide, water vapour, sulphur vapour, hydrogen, carbon dioxide, carbonmonoxide, argon, nitrogen and traces of carbon oxysulphide leaves thefurnace 16 through an outlet 22, typically at a temperature greater than1000° C. (and preferably at a temperature greater than 1400° C.). Atsuch temperatures, some of the components of the effluent gas stream arestill reacting with one another so it is difficult to specify theprecise composition of the gas mixture in the outlet 22. The gas streampasses from the outlet 22 directly into a waste heat boiler 24 or otherform of heat exchanger in which it is cooled to a temperature typicallyin the range of 300° C. to 500° C. During the passage of the gas streamthrough the waste heat boiler 24, there is a tendency for some of thehydrogen to re-associate with sulphur vapour to form hydrogen sulphide.Cooled effluent gas stream flows from the waste heat boiler 24 into afurther refractory lined furnace 26.

A lance 28 is employed to supply further pure oxygen (or, lesspreferably, oxygen-enriched air containing at least 80% by volume ofoxygen) to the furnace 26. The same reactions take place as in the firstfurnace 16 with the exception of oxidation of ammonia and hydrocarbonsbecause the gaseous feed to the furnace 26 is free of these components.The rate of supplying oxygen to the lance 28 is typically from 16 to 22%of the stoichiometric rate required for total combustion to sulphurdioxide and water vapour of the hydrogen sulphide present in the secondfurnace 26. The percentage conversion achieved in the second furnace 26is less than that achieved in the first furnace 16 because the operatingtemperature tends to be lower and because the oxygen may also react withthe hydrogen present in the gas entering the second stage 26.

The resulting gas exits the furnace 26 through another waste heat boiler30 and flows to a sulphur condenser 32 in which sulphur is condensed ata temperature typically in the order of 130° C.

The condensed sulphur flows along a pipeline 34 to a sulphur pit (notshown) via a sulphur seal leg (not shown).

The sulphur vapour depleted gas stream leaving the sulphur condenser 32is characterised by a high hydrogen sulphide to sulphur dioxide moleratio, typically in the range 4:1 to 10:1.

The sulphur vapour depleted gas stream flows through a reheater 36 inwhich it is reheated from its condensation temperature (typically in theorder of 130° C.) to a temperature in the order of 300° C. by indirectheat exchange with hot gas or direct heat exchange with a reducing gasgenerator (not shown).

The resultant heated gas stream passes to a catalytic reduction(hydrogenation) reactor 40 in which all the sulphur dioxide and residualtraces of sulphur vapour are reduced by hydrogen to hydrogen sulphideover a suitable catalyst. The catalyst may, for example, include a mixedcobalt-molybdenum oxide. In addition to the reaction between sulphurdioxide and hydrogen to form hydrogen sulphide and water vapour and thereaction between hydrogen and any sulphur vapour to form hydrogensulphide, other reactions can take place in the catalytic reductionreactor 40. In particular, any carbon monoxide present reacts with watervapour to form hydrogen and carbon dioxide. Further, at least 90% butnot all of any carbon oxysulphide present in the reheated furthersulphur vapour depleted gas stream is hydrolysed to carbon dioxide andhydrogen sulphide. Similarly, any carbon disulphide present ishydrolysed to carbon dioxide and hydrogen sulphide.

It is important to ensure that the reduction of sulphur dioxide and anyresidual sulphur goes to completion in the reactor 40. Otherwise therewill be a tendency for sulphur to deposit in downstream parts of theplant. Sometimes, there is sufficient hydrogen present in the sulphurvapour depleted gas stream for the reduction reactions to go tocompletion. In any event, it is preferred to have available a pipeline42 for the addition of external hydrogen in the event either of atemporary reduction of the hydrogen concentration in the gas mixtureentering the reactor 40 to a level at which complete reduction of thesulphur dioxide might be jeopardised or if there is an inadequatehydrogen concentration in the sulphur vapour depleted gas stream. Theexternal hydrogen may be generated on site by, for example, partialoxidation of hydrocarbon, preferably using pure oxygen oroxygen-enriched air as the oxidant, or, in conjunction with carbonmonoxide, by a reducing gas generator using air, oxygen-enriched air, orpure oxygen as the oxidant.

If desired, the catalytic reduction reactor 40 may be provided with acooling coil (not shown) in which a coolant, e.g. steam, may be passedin the event of there being an excessive generation of heat in thecatalyst, or, alternatively, steam can be added directly.

The reduced gas stream, now consisting essentially of hydrogen sulphide,hydrogen, water vapour, carbon dioxide, nitrogen and argon, leaves thereactor 40 and flows through a heat exchanger 46 in which it is cooledto a temperature in the range of 100° C. to 200° C. (e.g. 150° C.) byindirect heat exchange of water and/or steam. The thus cooled gas streamflows through one or more eductors 48 in parallel with one another. Inthe eductors 48 the cooled, reduced, gas stream is mixed withsuperheated, pressurised steam supplied via a pipeline 50. Typically,the furnaces 16 and 26 are operated at a pressure in the range of 1.2 to2 bar. The Claus plant 6 shown in FIG. 1 is operated at a similarpressure. The pressure of the reduced gas stream can be raisedsufficiently by this addition of stream to ensure passage of allnecessary gas to the Claus plant 6 by appropriate choice of the flowrate and supply pressure (and hence temperature) of the steam in thepipeline 50. Preferably, the steam is supplied at a pressure in therange of 10 to 50 bar and a corresponding temperature greater than 100°C. but less than 265° C.

The reduced gas stream, having been mixed with the steam is introducedinto a desuperheating, direct contact, quench tower 52. In the quenchtower 52, the gas stream flows upwardly and comes into contact with adescending stream of water. The reduced gas stream is thus cooled andmost (preferably more than 85%) of its water vapour content iscondensed, the condensate entering the descending liquid stream. Thecondensate includes the steam added to the reduced gas mixtures in theeductors 48. The quench tower 52 preferably contains a random orstructured packing (not shown) so as to facilitate mass transfer betweenthe ascending vapour and descending liquid. As a result, a watervapour-depleted gas stream is formed. The water exiting the bottom ofthe quench tower 52 is recirculated by means of a pump 54 and is cooledin a cooler 56 upstream of being reintroduced into the top of the quenchtower 52. Excess water is removed through an outlet 58 and is sent to asour water stripper (not shown) in order to remove its hydrogen sulphidecontent.

The resulting water vapour depleted gas stream, which passes out of thetower 52 through an outlet 60, typically contains in the order of atleast 40% by volume of hydrogen sulphide and therefore makes a suitablefeedstock for treatment in a conventional Claus plant.

The water vapour depleted gas streams leaves the top of the quench tower52 typically at a temperature in the range of 30° C. to 50° C. and issent to the Claus plant 6 shown in FIG. 5 of the accompanying drawings.Referring to FIG. 5, the water vapour depleted gas stream is received bya burner 72 firing into a further refractory-lined furnace 70. Theburner 72 may also receive a second feed gas stream which may be of thesame composition as or of a different composition from that of the feedgas stream to the furnace 16. For maximum uprating of the Claus plant 6,however, all the feed gas to it comes from the quench tower 52. Theburner 72 additionally receives a stream of air, oxygen-enriched air oroxygen through a pipeline 68.

In an oil refinery, there are various different strategies for selectingthe composition of the first and second feed gas streams. Generally inan oil refinery, there are one or more sources of amine gas, whichtypically contains more than 70% by volume of hydrogen sulphide but isfree of ammonia, and one or more sources of sour water stripper gas,which typically contains approximately equal proportions of watervapour, hydrogen sulphide and ammonia. One strategy is simply to mix theamine gas with the sour water stripper gas to obtain the samecomposition for both the first feed gas stream and the second feed gasstream. One problem that sometimes arises in Claus plant in that ofeffecting complete destruction of ammonia. If the ammonia is notcompletely destroyed it can poison or react with downstream Clauscatalysts. Particularly if air unenriched in oxygen is employed tosupport combustion in the further furnace 70, it is desirable that agreater proportion of the ammonia to be destroyed finds its way to thefirst feed gas stream rather than the second feed gas stream. This isbecause the relatively high oxygen mole fraction of the gas used tosupport combustion in the furnace 16 facilitates the creation of highflame temperatures which favour destruction of ammonia. Accordingly, itis often most preferred that all the sour water stripper gas is used informing the first feed gas stream. Typically, some of the amine gas ismixed with the sour water gas or supplied separately therefrom to theburner 14 that fires into the furnace 16. Any remainder of the amine gasis typically sent to the burner 72 as the second feed gas stream.

The Claus plant shown in FIG. 5 may be in essence a conventional Clausplant which is operated in essentially a conventional manner. Thereactions that take place in the furnace 70 are analogous to those thattake place in the furnace 16 shown in FIG. 2 and need not be describedfurther herein. A resulting gas stream having essentially the samecomponents as the effluent gas stream from the furnace 16 shown in FIG.2 leaves the furnace 70 through an outlet 74. (Typically, if a lowerflame temperature is attained in the furnace 70 than in the furnace 16,the gas mixture leaving through the outlet 74 will contain a lower molefraction of hydrogen than the corresponding gas leaving the furnace 16.)The effluent gas passes from the outlet 74 to a waste heat boiler 76 orother heat exchanger in which it is cooled by heat exchange with steamor other coolant. The resultant cooled gas stream typically leaves thewaste heat boiler 76 at a temperature in the range of 250° C. to 400° C.

The cooled effluent gas stream passes from the waste heat boiler 76 to afurther sulphur condenser 78 in which the effluent gas further cooled toa temperature in the range of 120° C. to 160° C. and in which thesulphur vapour is condensed and is extracted via an outlet 80. Theresulting liquid sulphur is typically passed to a sulphur seal pit (notshown). One particularly important difference between the operation ofthe furnace 16, on the one hand, and of the furnace 70 on the otherhand, is whereas at the outlet from the sulphur condenser 32 the moleratio of hydrogen sulphide to sulphur dioxide in the sulphur depletedgas stream is at least 4:1, the corresponding ratio at the outlet fromthe condenser 78 is in the order of 2:1. The sulphur vapour depleted gasstream flows from the further sulphur condenser 78 through threesuccessive catalytic Claus stages 84, 86 and 88. Each of the stages 84,86 and 88, in accordance with the general practice in the art, comprisesa train of units comprising, in sequence, a reheater (not shown) toraise the temperature of the gas mixture to a temperature suitable forcatalytic reaction between hydrogen sulphide and sulphur dioxide (e.g. atemperature in the range of 200° C. to 350° C.), a catalytic reactor(not shown) in which hydrogen sulphide reacts with sulphur dioxide toform sulphur vapour and water vapour, and a yet further sulphurcondenser (not shown).

The gas mixture leaving the most downstream catalytic stage 88 may besubjected to any one of a number of known treatments for rendering Clausprocess effluent suitable for discharge to the atmosphere. For example,the gas mixture may flow to the tail gas clean up plant 8 shown in FIG.6 of the accompanying drawings. With reference to FIG. 6, the gasmixture may pass to a reactor 90 in which the components present in thegas mixture are subjected to hydrolysis and hydrogenation. In thereactor 90, most of the residual carbon oxysulphide and carbondisulphide are hydrolysed over a catalyst, for example aluminaimpregnated with cobalt and molybdenum oxides to produce hydrogensulphide and carbon dioxide. At the same time, residual elementalsulphur and sulphur dioxide are hydrogenated to hydrogen sulphide. Thehydrolysis and hydrogenation typically take place at a temperature inthe range of 300° C. to 350° C. A resulting gas mixture comprising ofhydrogen sulphide, nitrogen, carbon dioxide, water vapour and hydrogenleaves the reactor 90 and flows first to a water condensation unit 92and then to a separate unit 94 in which hydrogen sulphide is separatedby means of absorption in a selective absorbent such asmethyldiethylamine. If desired, the hydrogen sulphide may be recycledto, for example, the furnace 70.

The plant shown in FIG. 1 is able to achieve more than 99.5% andtypically more than 99.7% conversion of hydrogen sulphide to sulphur. Byretrofitting the units 4 shown in FIG. 2 to the Claus plant 6 shown inFIG. 5, its capacity may be more than doubled without any loss ofconversion, or even with a gain in conversion

In FIG. 3, there is shown a modification to the arrangement of unitsdepicted in FIG. 2. There is located intermediate the waste heat boiler24 and the second furnace 26, a sulphur condenser 300 and a reheater302. The sulphur condenser 300 condenses sulphur at a temperature in theorder of 135° C. This step helps to enhance conversion of hydrogensulphide to sulphur vapour in the furnace 26 because the removal of thesulphur vapour has the effect of drawing forward the Claus reaction inthe second furnace 26. The reheater 302 raises the temperature of theresulting sulphur-depleted gas stream to a temperature in the order of500° C. such that autogenous combustion of hydrogen sulphide can takeplace in the furnace 300 and therefore the lance 28 can still be used tosupply oxygen to the second furnace 26.

In other respects, the arrangement and the operation of plant shown inFIG. 3 is essentially the same as that shown in FIG. 2.

The plant shown in FIG. 4 of the accompanying drawings and its operationare essentially the same as that shown in FIG. 3 with the exception thatthe reheater 302 is omitted. As a result a burner 402 with a separateoxygen inlet 404 is used instead of the lance 28. The burner 402typically has its own ignition and flame detection systems.

Although, referring again to FIG. 1, the series of units 2 has beendescribed as being retrofitted to the Claus plant 4 and the tail gasclean up plant 6, the entire plant may be assembled at the same time.

The method according to the present invention is now illustrated by thefollowing examples:

Referring again to FIG. 1, a subsisting Claus plant 4 and associatedtail gas clean up plant 6 treat a feed gas stream having the followingcomposition.

-   -   71.78 mole % H₂S    -   14.44 mole % H₂O    -   11.11 mole % NH₃    -   2.00 mole % CO₂    -   0.66 mole % C₂H₈

This feed gas stream is formed by mixing together amine gas with sourwater stripper gas in the ratio of two parts by volume of the former toone part by volume of the latter.

The feed gas stream is supplied to the Claus plant 4 at a rate of 100kmol/hr. This requires an air supply at a rate 202.73 kmol/hr.Accordingly, the furnace of the Claus plant has a volume sufficient tobe able to receive a total of 302.73 kmol/hour.

Three computer simulations (using a SULSIM 5 program) were performed inorder to evaluate uprating of the Claus plant by passing the feed gasinto each of the following:

-   -   A) a set 2 of units as shown in FIG. 4 of the accompanying        drawings;    -   B) a plant according to FIG. 4 of the drawings accompanying        EP-A-565 316 (i.e. a plant similar to FIG. 2 of the accompanying        drawings but omitting the furnace 26 and its associated waste        heat boiler).        The simulations were performed assuming that the oxygen feed to        the first furnace was 100% pure.

The results of the simulations are shown in Table 1 below, in which allflow rates are in kmol/hr. Based on previous practical experience of theoperation of oxygen-enhanced Claus furnaces, it is believed that theprogram underestimates the percentage conversion achieved in suchfurnaces. As a result the requirement for external hydrogen in thecatalytic hydrogenator may be overestimated. The results set out inTable 1 should therefore be taken as being confirmative of theoperability of the examples of the method according to the inventionrather than giving accurate operating data.

TABLE 1 A B Feed flow rate 100 100 Oxygen Flow rate to first furnace or30 32 (Case A only) to upstream combustion stage of the first furnaceExit temperature from first furnace or 1508° C. 1578° C. (Case A only)from upstream combustion stage of the first furnace Oxygen flow rate todownstream 10 — combustion stage of the first furnace (Case A only) Exittemperature from downstream 1041° C. — combustion stage of the firstfurnace (Case A only) H₂S to SO₂ mole ratio at inlet to 4.47:1 4.2:1catalytic hydrogenator External hydrogen supply rate to 5 2 catalytichydrogenator Total conversion (H₂S to S) 75.2% 57.5% H₂S in exit gas62.6 mole % 72.5 mole % Exit flow rate 28.4 41.9

These results show that the example of the method according to theinvention is able to achieve lower exit gas flow rates than the exampleof the method according to EP-A-565 316, thereby making possible agreater degree of uprating.

Based on the above results, the maximum feed rates of the acid gas tothe retro-fitted units 2 was calculated. The results of the calculationsare set out below:

A 460 kmol/hr B 275 kmol/hr

Thus, the method A according to the invention makes possible more than afourfold uprating of the Claus plant 4.

Even greater upratings are made possible if the Claus plant 4 isconverted to operate in accordance with EP-A-0 237 217 in which itssingle combustion furnace is replaced with two combustion furnaces inseries, in both of which oxygen is employed to support combustion. Nowthe maximum feed rates of acid gas become

A 810 kmol/hr B 600 kmol/hr

Various changes and modifications to the plant show in the drawings maybe made. For example, a considerable simplification of the plant 4 shownin FIG. 2, may be achieved by employing a plurality of passes in thewaste heat boiler 24. As a result the gas from the first pass can enterinto a chamber which acts as the furnace 26. The hot gas from thefurnace 26 flows through the second pass of the waste heat boiler 24 anddownstream thereof proceeds directly to the sulphur condenser 32. As aresult, the waste heat boiler 30 can be omitted altogether.

In another modification, the eductor or eductors 48 can be omitted andthe furnace 70 in the Claus plant 6 operated at a sufficiently lowerpressure than the furnaces 16 and 26 to obviate the need for a faninstead. A lower operating pressure can be created by reducing the totalgas flow to the furnace 70 on retrofitting the units 4. This isparticularly easy to do if the furnace 70 was before the retro-fitoperated with air as an oxidant. If oxygen is substituted for at leastsome of the air, the flow rate of oxidant may be reduced by an amountnecessary to give the desired lower operating pressure.

In a yet further modification, the eductor or eductors 48 can be omittedfrom the arrangements shown in FIGS. 2 to 4 and redeployed intermediatethe reactor 90 and the condenser 92 in the tail gas clean up plant shownin FIG. 6.

Finally, a catalytic; selective oxidation stage in which hydrogensulphide is selectively reacted with oxygen to form sulphur vapour andwater vapour over a selective oxidation catalyst can be substituted foror added to any of the catalytic Claus stages in the method andapparatus according to the present invention, particularly the mostdownstream catalytic Claus stage in the plant 6 shown in FIG. 5 of theaccompanying drawings. Selective oxidation catalysts are well known inthe art. Such a selective oxidation stage may be particularly useful asthe final catalytic stage in the plant 6 if the tail gas clean up unit 8is omitted from the arrangement shown in FIG. 1 because it helps toenhance the conversion of hydrogen sulphide to sulphur.

1. A method of treating a feed gas stream comprising hydrogen sulphide,comprising the steps of: a) in at least two furnaces in series oxidisingto sulphur dioxide a part of the hydrogen sulphide content of at leastone feed gas stream comprising hydrogen sulphide and reacting thusformed sulphur dioxide with residual hydrogen sulphide to form sulphurvapour and water vapour; b) withdrawing from the downstream furnace apartially reacted gas stream including sulphur vapour, water vapour,residual hydrogen sulphide and residual sulphur dioxide; c) extractingin a sulphur condenser, sulphur vapour from the partially reacted gasstream so as to form a sulphur vapour depleted gas stream; d)catalytically reducing with hydrogen to hydrogen sulphide the sulphurdioxide and any sulphur vapour content of the sulphur vapour depletedgas stream so as to form a reduced gas stream; e) extracting watervapour by condensation from the reduced gas stream so as to form a watervapour depleted gas stream; and f) feeding without recycle the watervapour depleted gas stream to at least one Claus plant for recoveringsulphur from hydrogen sulphide comprising at least one further furnacefor the oxidation of hydrogen sulphide to sulphur dioxide and reactionof resulting sulphur dioxide with residual hydrogen sulphide, a firstfurther sulphur condenser, and a plurality of stages of catalyticreaction, there being a second further sulphur condenser downstream ofeach stage of said catalytic reaction, and thereby extracting furthersulphur vapour wherein: a gas containing at least 80% by volume ofoxygen molecules is employed to support combustion in step a), the watervapour depleted gas stream contains at least 40% by volume of hydrogensulphide, and in step f) the catalytic reaction is a Claus reactionbetween hydrogen sulphide and sulphur dioxide or a selective oxidationof hydrogen sulphide to sulphur, or both reactions.
 2. A methodaccording to claim 1, wherein in step f) each stage of catalyticreaction is a Claus reaction between hydrogen sulphide and sulphurdioxide.
 3. A method according to claim 1, in which the mole ratio ofhydrogen sulphide to sulphur vapour in the sulphur vapour depleted gasstream is in the range of 4:1 to 10:1.
 4. A method according to claim 1,wherein the flow rate of oxygen to the upstream furnace in step a) is inthe range of M to N where M=(0.8 a+b+0.16c) and N=(a+b+0.22c) where: a)is the stoichiometric rate of supply of oxygen for the completeoxidation to nitrogen and water vapour of any ammonia in the feed gasstream; b) is the stoichiometric rate of supply of oxygen for thecomplete combustion of any hydrocarbon(s) present in the feed gasstream; and c) is the stoichiometric rate of oxygen supply required forthe complete oxidation to sulphur dioxide and water vapour of hydrogensulphide in the feed gas stream.
 5. A method according to claim 1wherein two furnaces are used, wherein the rate of supply of oxygen tothe downstream furnace in step a) is in the range P to Q. where P=0.8d+e+0.16f and Q=d+e+0.22f, where: d) is the stoichiometric rate ofoxygen required for the total oxidation to nitrogen and water vapour ofany ammonia entering the downstream furnace; e) is the rate of oxygensupply required for the total combustion to carbon dioxide and watervapour of any hydrocarbons entering the downstream furnace and f) is thestoichiometric rate of oxygen supply required for the completecombustion of hydrogen sulphide entering the downstream furnace to watervapour and sulphur dioxide.
 6. A method according to claim 1, whereinthe water condensation step e) is performed by direct contact of thereduced gas stream with water, the reduced gas stream being cooled byindirect heat exchange intermediate the catalytic reduction step d) andthe water condensation step e).
 7. A method according to claim 1,wherein from 80 to 90% of the hydrogen sulphide entering step a) isconverted to sulphur vapour upstream of the catalytic reduction step d).